In recent years, optical disks have been used in a variety of applications, such as recording or storing of images, sounds, or information. In particular, a recordable optical disk is expected to be used in applications, such as authoring of video or audio contents (i.e., a trial piece for mass production), or it is expected to be used as a medium on which a general user can record audio or video information.
It is preferred that the recordable optical disk has the same form as that of a reproduction-only optical disk, as in the case of a CD-R, i.e., the format of information recorded on a CD-R is the same as the format of information stored on a reproduction-only CD. The reason for this is that when the recordable optical disk and the reproduction-only optical disk use the same format, information recorded on the recordable optical disk can be reproduced by using a reproduction-only optical disk apparatus (e.g., a CD player) as it is.
This is not only a matter of cost of the optical disk apparatus but also a matter of importance to enhancement of content popularity. Specifically, the reproduction-only optical disk, such as a CD, has information pits preformed in a substrate thereof and such pits are usually formed by injecting resin into a die under high pressure. Accordingly, production facilities are large in scale, and thus the optical disks are supposed to be mass-produced.
On the other hand, the recordable optical disks, such as CD-Rs, are suitable for being produced in small quantities for various kinds of products. By using the same format for both the recordable optical disk and the reproduction-only optical disk, the optical disk apparatus can treat the recordable optical disk as a reproduction-only optical disk so as to reproduce information on the recordable optical disk, whereby it is possible to achieve quick adaptability to the diversity in the market.
In the case where an optical disk is used for authoring, even if system performance is verified using a recordable optical disk, when the format for reproduction-only optical disks to be mass-produced is different from that for the recordable optical disk, the same system performance cannot always be obtained. Thus, it is preferred that the reproduction-only disk and the recordable disk have the same format when being used for authoring.
However, when the same format is used for both the reproduction-only disk and the recordable disk for the aforementioned reasons, there arises a copyright problem. Specifically, it is possible to reproduce digital information on the reproduction-only disk so as to read the digital information bit-by-bit and sequentially record the digital information on the recordable disk (i.e., it is possible to make a so-called bit-by-bit copy), whereby the reproduction-only disk can be readily copied.
Thus, various kinds of disk format have been conventionally devised for the purpose of copyright protection. For example, Japanese Laid-Open Patent Publication No. 5-266576 discloses that when information is legally recorded (copied) on a disk, an invalid sector (a defect sector) is purposely provided on the disk. By applying this technology, it is possible to identify whether or not a target disk has been illegally copied from a reproduction-only disk, since no defect sector can be initially present on the reproduction-only disk. Moreover, Japanese Laid-Open Patent Publication No. 10-302404 discloses a method for preventing a bit-by-bit copy by cutting off a spiral groove preformed in a recordable disk in suitable places.
However, the aforementioned technologies are based on a premise that the reproduction-only disk and the recordable disk have different formats from each other.
The present invention has been made in view of the above problems and aims to provide: an optical disk medium which makes it possible to readily identify an illegal bit-by-bit copy even when both a reproduction-only disk and a recordable disk have the same format; a method for identifying an optical disk: and an optical disk apparatus.